The invention relates to a process for the preparation of organosilylalkylpolysulfanes.
It is known that organosilylalkylpolysulfanes, such as bis(3,3xe2x80x2-triethoxysilylpropyl)tetrasulfane (DE 2 141 159) and -disulfane, are employed as a silane adhesion promoter or reinforcing additive in rubber mixtures comprising oxidic fillers. The rubber mixtures are used, inter alia, for industrial rubber articles and for components of car tyres, in particular for treads (DE 2 141 159, DE 2 212 239, U.S. Pat. No. 3,978,103, U.S. Pat. No. 4,048,206, EP 819694).
It is furthermore known that the alkoxysilyl function, usually a trimethoxysilyl or triethoxysilyl group, reacts with the silanol groups of the filler, usually silicas, during preparation of the mixture and the silane is fixed on the filler surface in this way. The filler-rubber bond is then formed during the vulcanization process via the sulfur functionality of the fixed silane. The reactivity of the organosilylalkylpolysulfanes here depends decisively on the length of the polysulfane chain. Long chains with many sulfur atoms show a high reactivity. However, this high reactivity can lead to an undesirable premature reaction during processing. On the other hand, short-chain derivatives are significantly less reactive, but can be activated in a controlled manner via addition of additional elemental sulfur at a later point in time in the production process. This capacity for controlled activation of the compounds leads to a more economical production of rubber articles and to greater processing reliability. Organosilylalklyldisulfanes with a high disulfane content have particular advantages (EP 732362, L. Panzer, American Chem. Soc., Rubber Div. Meeting 1997).
It is furthermore known that organosilylalkylpolysulfanes with a reduced polysulfane chain length are prepared from the corresponding long-chain organosilylalkylpolysulfanes. EP 0773224 discloses a process in which organosilylalkylpolysulfanes are broken down to the corresponding disulfanes with the aid of cyanides, phosphanes or sulfites. In EP 0845472 and WO 97/48264, organophosphorus(III) compounds (inter alia phosphites and Pxe2x80x94N compounds) are used to reduce the polysulfane chains.
These processes have the disadvantage that for each molar equivalent of sulfur removed from the organosilylalkylpolysulfane, one molar equivalent of thiocyanate, organophosphorus(V) sulfide or thiosulfate is formed as a by-product.
EP 0894803 discloses a process in which the thiocyanate formed in the desulfurization with cyanide is reacted with an organosilylalkyl halide to give an organosilylalkyl thiocyanate, which is also reactive in the rubber.
A disadvantage of this process is that a mixture of an organosilylalkyldisulfane and an organosilylalkyl thiocyanate is obtained.
EP 0908463 and EP 0937732 furthermore disclose processes for reducing the sulfur chain length in organosilylalkylpolysulfanes in which the polysulfanes mentioned are reacted with an anhydrous or almost anhydrous ionic sulfide and then with organosilylalkyl halides.
A disadvantage of this process is the formation of considerable amounts of the by-product organosilylalkylmonosulfane, which cannot react with the rubber matrix. Products from this process are thus distinguished by a low content of active compound.
The object of the invention is to provide an alternative process for the preparation of organosilylalkylpolysulfane in which the amount of by-products which have to be disposed of is low.
The invention provides a process for the preparation of organosilylalkylpolysulfanes of the general formula I
(R1R2R3SiR4)2Sxxe2x80x83xe2x80x83(I)
in which the symbols denote
R1, R2, R3: which are identical or different from one another, branched and unbranched alkyl and/or alkoxy groups having a chain length of 1-8 C atoms, preferably 1-3 C atoms, aryl radicals, in particular phenyl, toluyl, benzyl, at least one alkoxy group being present;
R4 divalent alkylene radical having a chain length of 1-8 C atoms, such as, for example, methylene, ethylene, i-propylene, preferably n-propylene, i-butylene, 2-methylpropylene, n-butylene, n-pentylene, 2-methylbutylene, 3-methylbutylene, n-pentylene, 1,3-dimethylpropylene or 2,3-dimethylpropylene, preferably 1 to 4 C atoms, or
xe2x80x94(CH2)nxe2x80x94C6H4xe2x80x94(CH2)nxe2x80x94 where n=1-4,
x: number xe2x89xa71, preferably between 2 and 3,
xe2x80x83which is characterized in that organosilylalkylpolysulfane of the general formula II
(R1R2R3SiR4)2Syxe2x80x83xe2x80x83(II)
xe2x80x83in which
R1, R2, R3 and R4 have the abovementioned meaning and
y: number  greater than x, preferably between 2 and 6, particularly preferably between 3 and 5,
xe2x80x83is reacted with an ionic sulfide of the general formula III
M+2S2xe2x88x92xe2x80x83xe2x80x83(III),
xe2x80x83in which M+ represents an alkali metal cation, for example sodium or potassium cation, an ammonium ion, half an alkaline earth metal cation or half a zinc cation,
and an organosilylalkyl halide of the general formula IV
R1R2R3SiR4Xxe2x80x83xe2x80x83(IV)
xe2x80x83in which
R1, R2, R3 and R4 have the abovementioned meaning and
X: is chlorine bromine or iodine,
xe2x80x83the long-chain organosilylalkylpolysulfane of the general formula (II) and the organosilylalkyl halide of the general formula (IV) being initially introduced into the reaction vessel and the ionic sulfide of the general formula (III) being added to this solution in several portions.
Because of the susceptibility of the starting substances according to formula (II) and formula (IV) to hydrolysis, the sulfides of the general formula III can be anhydrous or almost anhydrous. The sulfide of the general formula III can contain a maximum of 10 wt. %, preferably 0-5 wt. %, particularly preferably 0-2 wt. % of water. Sulfides of the general formula III can be obtained by:
1. Reaction of alkali metal alcoholates with hydrogen sulfide (EP 705838).
2. Reaction of ammonia gas with hydrogen sulfide (DE 2648241).
3. Drying of alkali metal sulfide hydrates (JP 7228588, DE 19610281, DE 19651849).
It is irrelevant here whether the drying of the alkali metal sulfide hydrates is carried out azeotropically or by heating in vacuo. The ionic sulfide required can advantageously be prepared by the process described in DE 196 51849.
The ionic sulfide of the general formula (III) can be employed in the form of a solid, either as a ground powder or as platelets, such as are available in the case of commercially obtainable alkali metal sulfide hydrates, or in the form of a solution or suspension of the solid in an organic solvent.
All polar solvents in which the ionic sulfide of the general formula (III) is at least partly soluble and which do not react with the organosilicon compound of the general formula (II) can be employed as the polar organic solvent.
The molar ratios of the educts according to formulae (II) and (III) can depend on what average polysulfane chain length y is present in the starting compound and what average polysulfane chain length x is to be obtained in the end product.
The molar ratio between the ionic sulfide of the formula (III) and the organosilylalkyl halide of the formula (IV) can in turn depend on the active compound content of the ionic sulfide. It can be between 1.5 and 2.5 molar equivalents, preferably between 1.8 and 2.2 molar equivalents of organosilylalkyl halide of the formula (IV) per molar equivalent of ionic sulfide of the formula (III).
The reaction can be carried out with exclusion of air and water (moisture), in order to suppress or avoid to the greatest extent the formation of by-products. The reaction can be carried out at elevated temperature. It is not essential here for the process according to the invention whether, to achieve the reaction temperature, the reaction mixture must be heated externally or heats up by itself due to the exothermicity released. The reaction can be carried out at between room temperature and 200xc2x0 C., preferably between 40xc2x0 C. and the boiling temperature of the solvent employed. The reaction can be carried out under normal pressure, reduced or elevated pressure.
After the reaction, the ionic halide which has precipitated out can be filtered off and the solvent can be distilled off.
By this procedure, the organosilylalkylpolysulfanes of the formula (I) can be prepared without the troublesome formation of unreactive by-products which cannot be separated off.
The polysulfane chain lengths x in the formula (I) and y in the formula (II) are to be understood as mean values. The values x and y designate the average lengths of the polysulfane unit present in the product mixture.
Since the polysulfane chain length of the compound of the formula (II) is to be reduced in the reaction according to the invention, so that the compound of the formula (I) results, y must be  greater than x.
In a particularly preferred embodiment of the invention, the organosilylalkylpolysulfane of the general formula (II) can already be reacted with the ionic sulfide of the general formula (III) and the organosilylalkyl halide of the general formula (IV) during its formation (in situ). In this case the organosilylalkylpolysulfane of the general formula (II) prepared in situ from an organosilylalkyl halide of the general formula IV and an ionic polysulfide of the general formula V
M+2Sy2xe2x88x92xe2x80x83xe2x80x83(V)
in which M+ and y have the abovementioned meaning, can be reacted with an ionic sulfide of the general formula (III) and further organosilylalkyl halide of the general formula (IV). The reaction can be carried out in a polar organic solvent. The organosilylalkyl halide of the general formula (IV) necessary for the reaction can be heated with the ionic polysulfide of the general formula (V) and the ionic sulfide of the general formula (III) necessary for reducing the polysulfane chain length can be metered in at elevated temperature in several portions.
The content of sulfur in the ionic polysulfide of the general formula (V) can be identical to the average polysulfane chain length y of the organosilicon polysulfide of the formula (II) which can be formed intermediately. Ionic polysulfides of the general formula (V) which can be used are, preferably, sodium, potassium or ammonium polysulfide, particularly preferably sodium polysulfide. Because of the susceptibility of the starting substances according to formula (II) and formula (IV) to hydrolysis, the ionic polysulfides of the general formula (V) can be anhydrous or almost anhydrous. The polysulfide of the general formula (V) can contain a maximum of 10 wt. % of water, preferably 0-5 wt. %, particularly preferably 0-2 wt. %. Polysulfides of the general formula (V) can be obtained by:
1. Reaction between an anhydrous or almost anhydrous sulfide and sulfur (JP 7228588)
2. Reaction between elemental alkali metals and sulfur, either in the melt (U.S. Pat. No. 4,640,832) or in an inert solvent (DE 19819373, EP 949263, G. Brauer, Handbuch der praparativen anorganischen Chemie [Handbook of Preparative Inorganic Chemistry], 3 rd edition, Stuttgart 1975, volume 1, p. 376 ff)
3. Reaction between alcoholates and sulfur (U.S. Pat. No. 5,596,116)
4. Reaction between water-containing alkali metal sulfide and sulfur with subsequent drying (DE 19651849)
5. Reaction between alkali metal hydroxide and sulfur with subsequent drying (DE 19930495)
The ionic polysulfide according to formula (V) can be employed in the form of a solid (powder, granules) or in the form of a solution or suspension of the solid in an organic solvent, without affecting the success of the reaction.
In the particularly preferred embodiment of the invention, in which the organosilylalkylpolysulfane of the formula (II) is already reacted further to give the target compound of the formula (I) during its formation, the molar ratio between the ionic polysulfide of the formula (V) used for the preparation of the organosilylalkylpolysulfane of the formula (I) and the ionic sulfide according to formula (III) can be established such that the average sulfur content of this mixture approximately corresponds to the average sulfur chain length x in the target compound (I). The molar ratio between the ionic polysulfide of the formula (V) and the ionic sulfide of the formula (III) and the organosilylalkyl halide of the formula (IV) can in turn depend on the active compound content of the ionic polysulfide and sulfide. It can be between 1.5 and 2.5 molar equivalents, preferably between 1.8 and 2.2 molar equivalents of organosilylalkyl halide of the formula (IV) per molar equivalent of polysulfide and sulfide.